How can I create an L shape with two beveled sides?
You can do this with the bevel tool.

Start with a cube, in edit mode (TAB) select the edges you want to make round:



press CtrlB to use the bevel tool and set the parameters for Amount to 1 and Profile to 0.5



Remove doubles (press the Spacebar or press Ctrlv->Remouve Doubles)


To make the first shape you describe extrude these faces: 



For the second shape you'd need to make a cut before extruding: